GROWTH-RATES AND NUTRITION STATUS OF AN OPEN AND A CLOSED POPULATION OF RHODODENDRON-FERRUGINEUM L IN THE NORTHWESTERN ALPS (FRANCE)

This study evaluates the utility of cell compounds as indicators of nu trition status of plant populations. An overview of the soluble free a mino-acids, carbohydrates and P-compounds in the two year classes of l eaves from Rhododendron ferrugineum populations showing variable biolo gical performances, was drawn up using carbon-13 and phosphorus-31 nuc lear magnetic resonance spectroscopy (NMR). The results showed differe nces between the age of leaves and the degree of population closure. T he open population, which has the smallest growth rate and net primary productivity, had generally lower levels of aminoacids but higher lev els of carbohydrates and P-compounds. Neither the amounts of mineral n itrogen produced by the soils nor the total nitrogen concentrations in leaves differed significantly. Still we hypothesize that the nitrogen avail ability could be largely responsible for the variations observe d between the populations, as phosphorus analyses showed that the two populations did not suffer significant P deficiencies. Differences in vitality of R. ferrugineum may be explained by the fact that in an ope n population R. ferrugineum is forced into a situation of sharing nutr ients with other species while in a closed population it is the sole s pecies to exploit a specific pool of nutrients. Finally we observed th at the C-13- and P-31-NMR approach is more suitable for studying the n utrition status of plant populations under field conditions than the d etermination of the total amounts of different elementary nutrients.